How Do I Calculate Voltage Drop?
The voltage drop formula for copper and aluminum wire, the 3% recommendation, and why ampacity is a separate check.
Open the Voltage Drop Calculator
The formula
For a single-phase circuit: VD = 2 × K × I × L ÷ CM, where K is the conductor’s resistivity (about 12.9 for copper, 21.2 for aluminum, in ohm-circular-mils per foot), I is current in amps, L is one-way length in feet, and CM is the conductor area in circular mils. The factor 2 accounts for current flowing out and back. Three-phase circuits use √3 instead of 2.
A worked example
12 AWG copper has about 6,530 circular mils. At 20 A over 100 ft: 2 × 12.9 × 20 × 100 ÷ 6,530 ≈ 7.9 V. On a 120 V circuit, that is 6.6% — well above the 3% commonly recommended for branch circuits.
Voltage drop is not ampacity
Voltage drop is about delivering usable voltage over distance. Ampacity is about the conductor not overheating, and it is governed by the NEC based on insulation rating, terminals, temperature and installation. A wire must pass both checks. Have a qualified electrician confirm any installation.